Key-hole mining: engineering solutions towards zero-waste anodic electro-oxidation of green technology metals from sulphidic ores (ZERO-electro)

Lead Research Organisation: UNIVERSITY OF EXETER
Department Name: Camborne School of Mines


Metals are essential components of almost all modern technology. Amongst these are the emerging technologies on which we are depending to tackle the Climate Emergency: electric motors, batteries, transformers, photovoltaic panels and catalysts, just to name a few.

Consequently, demand for 'green technology metals' (including Ni, Cu, Pd and Co) is surging, and is projected over the next 25 years, to eclipse the total for all previous human history. Recycling can only deliver a fraction of the supply. Even for metals such as Co, for which it is as high as 70%, it only accounts for 30% of demand. For metals which are more difficult to recycle, including Se, In and V, it remains <1%. It is therefore clear that the continued health and prosperity of both humankind and the natural environment depend on a huge increase in sustainable metal mining this Century.

Despite such urgency, our methodology for the extraction of metals from the subsurface hasn't changed since the inception of metal mining which marked the beginning of the Bronze Age; we still "dig up" the raw materials. This results in environmental damage on a truly global scale. Therefore, whilst the metals extracted may be used to build 'green technologies' the nature of their extraction, via energy intensive digging, haulage and crushing, means that there is considerable "embedded carbon" in all metal products. This is hampering our ability to address the Climate Emergency.

In fact, the situation is presently worsening, because as the near-surface ore deposits are being exhausted we are resorting to digging deeper into the subsurface (>1km depth in some cases) to reach them. The massive energy consumption involved is raising the degree of embedded carbon in humanity's metal supply, just at the time when we need urgently to reduce it.

This is a global problem but also one which is important for the UK. Burgeoning demand for green technology metals coupled with various shifts in geopolitical conditions have dictated that metal mining is back on the UK political agenda. The prospect of a mining renaissance, however, has attracted scrutiny from the general public who have expressed concerns that it will compound and reproduce the social and ecological damage that has been associated with extractive activities in the past. Indeed, the high population density of the UK and Europe demands radical new thinking into what technology is appropriate for the extraction of our metal resources. We need radical new thinking in how we extract metals from the subsurface.

This project seeks an entirely new approach to metal mining. In particular we will investigate the use of electricity and a suitable electrolyte (liquid that can carry dissolved metal ions) to decompose a metal-bearing ore deposit (to yield the desired metal) whilst it remains buried in the subsurface. Fundamental electrochemical theory suggests that this may be possible only using only a modest energy supply (i.e. of the same order of magnitude as can be supplied using a modest-sized array of solar panels). The metal laden electrolyte fluid will then be pumped to the surface.

We anticipate that this new method would be particularly applicable for an important class of minerals that comprise metals bonded with reduced sulfur, known as the sulfides. These are noteworthy for their ability to conduct electricity, which is a critical requirement. The sulfides are widely regarded as the most important type of ore and currently supply approximately >80% of all Cu, >70% of all Co, >60% of all Ni, >95% of all Zn and >99% of all platinum group metals.

This project will provide the fundamental "proof of concept" data for this radically new approach to metal mining. We anticipate several technical challenges, however if we are successful, then we could unlock an entirely new sustainable future.


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Description Key Findings:
• The electro-oxidation of sulphide minerals in environmentally benign chloride based electrolytes has been shown to be feasible.

• The selective extraction of target metals is viable by control of leaching conditions, particularly in the case of copper from chalcopyrite.

• Non-linear potential profiles show promise in mitigating of passivation problems affecting sulphide oxidation.

Objective 1: Determine the dependence of sulfide stability and decomposition rate on the voltage across the sulfide-electrolyte interface:
The stability and electro-oxidation behaviour of various sulphide minerals has been investigated via cyclic voltammetry on ore samples from two sources. The first, from Cobres las Cruces, Spain, consists solely of sulphide minerals, predominantly pyrite, with smaller amounts of chalcopyrite and sphalerite. The second, from Kimberley, Western Australia possesses more complex mineralogy with the bulk being made up of sulphide minerals, mainly chalcopyrite, with some pyrite and a small amount of pentlandite, in addition to silicates including almandine, olivine and ilmenite, plus magnetite and quartz.
Cyclic voltammograms on Cobres las Cruces samples demonstrate the commencement of electro-oxidation at potentials of around 0.5 V vs SHE, attributed predominantly to the oxidation of chalcopyrite, with further rapid oxidation occurring at potentials positive to 0.8 V vs SHE, due to the decomposition of pyrite. The oxidation of sphalerite is known to commence at 0.45 V vs SHE but no clear anodic peak is observed on the voltammograms corresponding to this, likely due to the relatively minor amount present in the sample. The behaviour of the sample from Kimberley is more complex, with additional anodic peaks observed on the voltammograms at 0.8 V vs SHE, attributed to the partial oxidation of pentlandite, and at 1 V vs SHE, assigned to the oxidation of almandine.

Objective 2: Suppression of gangue mineral and non-target metal dissolution:
Following the results of cyclic voltammetry, constant potential leaching has been carried out in chloride based electrolytes at potentials between 0.6 V and 1.1 V vs SHE, and the resultant leachates analysed by ICP-OES, in order to identify suitable conditions and opportunities for the selective targeting of specific metals. On samples from both Cobres las Cruces and Kimberley, results have demonstrated good selectivity for copper extraction from chalcopyrite utilising certain experimental parameters, with up to nearly 90% of the extracted metal being copper under optimal conditions (further tests are required to fully identify the mechanics of this process). Increasing zinc extraction from sphalerite is observed across the potential range employed, although the amounts remain minor in relation to copper and iron.
Significant increases in iron extraction are observed for the Kimberley samples as the potential passes 0.7 V vs SHE. This is attributed to the partial oxidation of pentlandite releasing Fe2+ and forming an iron deficient passivation layer at these potentials. As the potential increases beyond 0.8 V vs SHE complete oxidation of pentlandite occurs and Ni2+ electrolyte content increases. Further increases in leached iron at potentials exceeding 0.8 V vs SHE occur due to the onset of irreversible pyrite oxidation. Additionally, aluminium is increasingly present in the leachates at potentials of 1 V vs SHE and above, signifying the oxidation of almandine.
Crucially, these results suggest that target metals may be extracted with good selectivity even in the presence of complex and mixed mineralogy, although further investigation is required to confirm this. The opportunity for selective sequential extraction of zinc from sphalerite and nickel from pentlandite also exists, although avoiding the unwanted extraction of iron during the latter may be difficult at the more positive potentials required.

Objective 3: Suppression of sulfide passivation during leaching:
ZERO-electro has focussed attention of chloride based electrolytes, with as close to neutral pH as possible, in an attempt to provide environmentally benign solvent options suitable for use in real world in situ applications. The effect of varying pH and Cl- concentration has been investigated via cyclic voltammetry on sulphide ores and increasing activity for anodic processes observed as pH increases. However, near neutral electrolytes are to be avoided due to the precipitation of metallic hydroxides. Increasing Cl- concentrations up also improve anodic activity, although as Cl- content approaches the limit of solubility oxidation is suppressed at higher potentials. This is believed to be due to the formation of an insoluble CuCl layer at these potentials, which has been observed by EDS after leaching experiments at 1 V vs SHE. Initial leaching experiments employing sinusoidal potential profiles show some promise is alleviating this passivation behaviour, and further tests utilising sinusoidal and pulsed potential profiles are to be undertaken to investigate this further.
Exploitation Route We are now looking to work with our collaborators and commercial partners on developing this mechanistic understanding into a process which is applicable for industry.
Sectors Environment

Description Impact Accelerator Account - ZERO-electro
Amount £24,818 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 02/2024 
End 03/2024
Description Collaboration with CSIRO Australia 
Organisation Commonwealth Scientific and Industrial Research Organisation
Country Australia 
Sector Public 
PI Contribution Dissemination of ZERO-electro research objectives in order to identify suitable field sites for samples
Collaborator Contribution Steve Barnes (CSIRO) has provided suitable samples
Impact Samples have been sent to us for testing.
Start Year 2023
Description Collaboration with FQM 
Organisation First Quantum Minerals
Country Canada 
Sector Private 
PI Contribution Discussion on ZERO-electro objectives
Collaborator Contribution Sharing of data, sending of samples
Impact We have received data to support the research and samples for testing.
Start Year 2023